Consider California's new mandate. The state's peak electricity demand is about 52,000 megawatts. Meeting the one-third target will require (if you oversimplify a bit) about 17,000 megawatts of renewable-energy capacity.

The math is simple: To have 8,500 megawatts of solar capacity, California would need at least 23 projects the size of Ivanpah, covering about 129 square miles, an area more than five times as large as Manhattan.

To have 8,500 megawatts of wind generation capacity, California would likely need to set aside an area equivalent to more than 70 Manhattans.

That's 1,806 square miles to cover 1/6th of the need of one state. Also, Not every state has windy alley or a mojave desert.